In a typical fractional distillation process, a feed stream containing at least first and second components is supplied to a tower. The tower is so constructed that rising vapors pass upward through layers of condensate on a series of plates. The vapor passes from one plate to the next above by bubbling under caps and out through the liquid on the plate. The less volatile (heavier) portions of the vapor condense in bubbling through the liquid on the plate, and excess liquid overflows the plate on which it was condensed and passes to the next lower plate through passages called downcomers. Ultimately the heavier portions flow to the bottom of the tower, and separation of the light and heavy fraction is thereby affected.
A substantial portion of the lighter component contained in a feedstream is removed from the distillation column as an overhead product and a substantial portion of the heavier component in the feedstream is removed from the distillation column as a bottoms product. Heat is generally supplied for the fractional distillation process either by preheating the feed to a desired temperature or by supplying heat directly to the tower.
Fractional distillation towers are also employed in many processes to make more complex multiple component separations. Complex separations are often performed in vacuum towers so that the boiling temperature is reduced sufficiently to prevent cracking of the material being distilled. A special class of fractionation processes are carried out in feedstock vacuum towers in which a distillation tower is divided into sections by one or more total drawoff trays. As used herein a total drawoff tray is a tray in a tower which has its downcomer passage sealed and from which all the liquid fraction is removed as a sidestream. Feedstock vacuum towers are used for distilling broad boiling range hydrocarbon streams, for example, from the residue of a crude oil distillation process. These columns separate out an overhead vapor fraction, a bottoms liquid fraction and at least one intermediate boiling range or so called sidecut fraction. The sidecut fraction which is withdrawn in a sidestream can be utilized as a feedstock for another process unit such as a cat cracker.
In this sectionalized tower the top section is refluxed with a portion of the overhead liquid product, and each of the lower sections is refluxed with a portion of the respective sidestream withdrawn from that section. Each sidestream is withdrawn laterally in liquid form at different levels within the tower and each sidestream is characterized by its cut point temperature and associated boiling point range. As used herein the cut point temperature is the boiling temperature division between cuts of the feedstock.
A preheated hydrocarbon mixture is fed into the feedstock vacuum tower at a predetermined flow rate and then abruptly expanded, thus causing vaporization of the major fraction of the hydrocarbon mixture. The vapors obtained rise within the column and are washed by down flowing liquid reflux which was previously fed back to the top of each section of the tower. As previously stated one or more sidestreams are withdrawn from the feedstock vacuum tower.
In the past it has been common practice to control separations between the cut point temperature of the sidestreams by manipulating internal reflux flow rates in the column in response to a load curve, wherein the load curve is utilized to predict the required internal reflux as a function of the feed rate. However with control systems based directly on internal reflux, the external refluxes must be manipulated slowly to avoid interactions between product quantity controllers and external reflux controllers and to avoid interactions between different reflux controllers. Control schemes based on load curves are satisfactory if the magnitude and frequency of disturbance caused for example, by changes in feed composition, feed temperature, tower pressure, etc., are low. However, when the frequency of disturbances is high, the slow manipulation of the external reflux causes disturbances to propagate in the distillation tower which leads to unstable operation in general and in particular leads to off specification conditions for the sidedraw products.
It is thus an object of this invention to control a feedstock vacuum tower so as to maintain a desired separation between cut points of fractions withdrawn from the tower in order to maximize the yield of a more valuable fraction.
It is a further object of this invention to base manipulation of external reflux streams on measured temperatures and pressures found in the tower so as to stabilize the liquid loads in each section of the tower.